Write concern basics in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the effect of write concern { w: 0 }?

Consider a MongoDB insert operation with the write concern set to { w: 0 }. What will be the behavior of this operation?

A The operation returns immediately without waiting for any acknowledgment from the server.

B The operation waits for acknowledgment from the primary only.

C The operation waits for acknowledgment from all replica set members.

D The operation fails immediately with an error.

💡 Hint

Think about what w: 0 means in terms of server acknowledgment.

Write concern { w: 0 } means the client does not wait for any acknowledgment from the server. The operation is sent and the client continues immediately without confirmation.

❓ query_result

intermediate

What does write concern { w: 'majority' } guarantee?

In MongoDB, if you set the write concern to { w: 'majority' }, what does this guarantee about the write operation?

A The write is acknowledged after being written to all members, including arbiters.

B The write is acknowledged only by the primary node.

C The write is acknowledged immediately without waiting.

D The write is acknowledged after being written to a majority of voting replica set members.

💡 Hint

Consider what 'majority' means in a replica set context.

Write concern { w: 'majority' } ensures the write is acknowledged only after a majority of voting replica set members have written the data, providing stronger durability guarantees.

📝 Syntax

advanced

Identify the invalid write concern option

Which of the following write concern options is invalid and will cause a syntax or runtime error in MongoDB?

A { w: 2, wtimeout: 5000 }

B { w: -1, j: false }

C { w: 'majority', j: true }

D { w: 1, wtimeout: 0 }

💡 Hint

Check the valid range and types for the w option.

The w option must be a positive integer or the string 'majority'. Negative values like -1 are invalid and cause errors.

🧠 Conceptual

advanced

What happens if write concern timeout is reached?

If a write operation uses a write concern with a wtimeout set, and the timeout is reached before the write is acknowledged, what is the outcome?

A The write operation waits indefinitely until acknowledged.

B The write operation is aborted and rolled back automatically.

C The write operation completes but the client receives a timeout error.

D The write operation is acknowledged immediately ignoring the timeout.

💡 Hint

Think about what a timeout means for client acknowledgment.

If the wtimeout is reached before acknowledgment, the write may still complete on the server, but the client receives a timeout error indicating the acknowledgment was not received in time.

🔧 Debug

expert

Why does this write concern cause a write failure?

A MongoDB write operation uses the write concern { w: 3, wtimeout: 1000 } on a replica set with only 2 voting members. What error will occur and why?

A The operation fails with a write concern error because the requested w is higher than the number of voting members.

B The operation succeeds immediately because wtimeout is set to 1000 ms.

C The operation fails with a syntax error due to invalid wtimeout value.

D The operation ignores the write concern and writes only to the primary.

💡 Hint

Consider the relationship between w and the number of voting members in the replica set.

Write concern w cannot be greater than the number of voting members in the replica set. Requesting w: 3 on a 2-member set causes a write concern error.

Practice

1.

What does write concern in MongoDB control?

easy

A. The size of the database files

B. The speed of reading data from the database

C. The number of users connected to the database

D. How sure MongoDB is that your data is saved

Solution

1. Step 1: Understand the role of write concern

   Write concern defines the level of acknowledgment requested from MongoDB when writing data.

2. Step 2: Identify what write concern controls

   It controls how sure the database is that the data has been saved successfully.

3. Final Answer:

   How sure MongoDB is that your data is saved -> Option D

4. Quick Check:

   Write concern = Data save confirmation [OK]

Hint: Write concern = data save confirmation level [OK]

Common Mistakes:

• Confusing write concern with read speed
• Thinking it controls database size
• Assuming it manages user connections

2.

Which of the following is the correct way to set a write concern of w: 1 in a MongoDB insert operation?

db.collection.insertOne({name: 'Alice'}, {writeConcern: ???})

easy

A. {w: 1}

B. {w: 'majority'}

C. {w: 0}

D. {w: true}

Solution

1. Step 1: Recall write concern syntax

   Write concern is set as an object with key w and a value indicating the level.

2. Step 2: Match the correct value for w: 1

   The correct syntax is {w: 1} to wait for acknowledgment from one server.

3. Final Answer:

   {w: 1} -> Option A

4. Quick Check:

   Write concern syntax = {w: 1} [OK]

Hint: Use {w: 1} to wait for one server acknowledgment [OK]

Common Mistakes:

• Using string 'true' instead of number 1
• Confusing 'majority' with numeric 1
• Setting w to 0 which means no acknowledgment

3.

What will happen if you run this MongoDB command?

db.orders.insertOne({item: 'book'}, {writeConcern: {w: 0}})

medium

A. The insert does not wait for any confirmation

B. The insert waits for confirmation from the server

C. The insert waits for confirmation from majority of servers

D. The insert throws a syntax error

Solution

1. Step 1: Understand writeConcern w: 0 meaning

   Setting w: 0 means no acknowledgment is required from the server.

2. Step 2: Predict behavior of insertOne with w: 0

   The insert operation will send data but not wait for any confirmation, so it returns immediately.

3. Final Answer:

   The insert does not wait for any confirmation -> Option A

4. Quick Check:

   w: 0 means no wait for confirmation [OK]

Hint: w: 0 means fire-and-forget, no wait [OK]

Common Mistakes:

• Thinking w: 0 waits for server confirmation
• Assuming syntax error due to w: 0
• Confusing w: 0 with majority write concern

4.

Identify the error in this MongoDB write concern usage:

db.users.insertOne({name: 'Bob'}, {writeConcern: {w: 'two'}})

medium

A. The write concern should be set as a string without quotes

B. The value 'two' is invalid for write concern w

C. The insertOne method does not accept write concern

D. The document format is incorrect

Solution

1. Step 1: Check valid values for write concern w

   Write concern w accepts numbers or 'majority', not arbitrary strings like 'two'.

2. Step 2: Identify the error in the given code

   Using 'two' is invalid and will cause an error.

3. Final Answer:

   The value 'two' is invalid for write concern w -> Option B

4. Quick Check:

   Write concern w must be number or 'majority' [OK]

Hint: Write concern w must be number or 'majority' only [OK]

Common Mistakes:

• Using invalid string values for w
• Thinking writeConcern is not allowed in insertOne
• Confusing quotes usage in write concern

5.

You want to ensure your MongoDB write operation waits for confirmation from the majority of replica set members but also times out if it takes more than 5 seconds. Which write concern option should you use?

hard

A. {w: 0, wtimeout: 5000}

B. {w: 1, wtimeout: 5000}

C. {w: 'majority', wtimeout: 5000}

D. {w: 'majority', wtimeout: 0}

Solution

1. Step 1: Understand write concern for majority

   To wait for majority confirmation, w must be set to 'majority'.

2. Step 2: Add timeout for waiting

   Use wtimeout to specify max wait time in milliseconds; 5000 means 5 seconds.

3. Step 3: Combine options correctly

   The correct option is {w: 'majority', wtimeout: 5000} to wait for majority with 5 seconds timeout.

4. Final Answer:

   {w: 'majority', wtimeout: 5000} -> Option C

5. Quick Check:

   Majority + 5s timeout = {w: 'majority', wtimeout: 5000} [OK]

Hint: Use w: 'majority' with wtimeout in ms for timeout [OK]

Common Mistakes:

• Using w: 1 instead of 'majority' for majority confirmation
• Setting wtimeout to 0 which means no timeout
• Using w: 0 which disables waiting